Hitherto, in general, an electronic clock including analog indication means has hands that are driven by a stepper motor (also referred to as “stepping motor” and “pulse motor”). This stepper motor includes a stator to be magnetized by a coil, and a rotor that is a disc-shaped rotary member magnetized into two poles. For example, the stepper motor is driven for each second to indicate the time with the hands.
Further, the electronic clock obtaining multiple functions in recent years is capable of presenting indication depending on various situations by revolving the hands not only clockwise but also counterclockwise with use of a forward/reverse stepper motor capable of obtaining forward rotation and reverse rotation. Further, a wristwatch is required to be reduced in size and thickness and also in power consumption. There has been commercialized an electronic clock including load compensation means for detecting a rotational state of the rotor to supply an optimum drive pulse based on load fluctuations and the like of the stepper motor in order to improve the drive efficiency of the stepper motor.
There has been proposed such a technology of driving a reversible stepper motor for achieving multiple functions, reduction in size and thickness, and reduction in power consumption of the electronic clock, and also capable of rotating the forward/reverse stepper motor at high speed (for example, see Patent Literature 1).
The reversible stepper motor of Patent Literature 1 includes a rotor that is magnetized to an S-pole and an N-pole in a radial direction of the rotor, a first stator magnetic-pole portion and a second stator magnetic-pole portion that are formed to be substantially opposed to each other through intermediation of the rotor, a third stator magnetic-pole portion formed between the first and second stator magnetic-pole portions, a coil A to be magnetically coupled to the first stator magnetic-pole portion and the third stator magnetic-pole portion, and a coil B to be magnetically coupled to the second stator magnetic-pole portion and the third stator magnetic-pole portion.
Then, while one of the coil A and the coil B is driven, the other of the coil A and the coil B is operated as a detection coil for detecting a counter-electromotive current that is generated in accordance with a rotational angle of the rotor. That is, the reversible (forward/reverse) stepper motor includes two drive coils with respect to one rotor.
Such a two-coil forward/reverse stepper motor can be driven for forward rotation and reverse rotation with a drive waveform of the same timing, as compared to the related-art one-coil forward/reverse stepper motor. Therefore, there are advantages in that the drive speed in forward rotation is equal to that in reverse rotation, and thus high-speed drive is enabled.
Further, as another technology of the electronic clock using the stepper motor, there has been proposed an electronic clock including impact compensation means capable of preventing irregular motions of hands due to a mechanical impact applied from the outside (for example, see Patent Literature 2).
In this case, a wristwatch of an analog indication type using hands is required to be reduced in size as a matter of course in order to wear the wristwatch on the arm. As a result, a problem of visibility arises due to the small hands (second hand, minute hand, hour hand, and the like). In order to improve the visibility of the wristwatch of the analog indication type, it is conceivable to use thick hands to facilitate visualization, for example. However, the thick hands cause increase in weight, and there has been a problem in that, even when a small impact is received by the hands from the outside, the impact is transmitted to the stepper motor to move (rotate) the rotor of the stepper motor, resulting in deviation of the indicated time.
In order to solve this problem, the holding force of the stepper motor may be increased, but when the holding force is increased, the drive power of the stepper motor is increased. Thus, this method cannot be adopted from the viewpoints of downsizing of the electronic clock and battery life.
The electronic clock of Patent Literature 2 includes the impact compensation means for detecting such an impact applied from the outside based on a counter-electromotive current generated from the stepper motor to output a lock pulse for braking the stepper motor when the impact is detected, to thereby prevent irregular motions of the hands. With this, the stepper motor can be braked when the impact is detected. Therefore, for example, the hands can be upsized to improve the visibility of the indicated time. Further, the restrictions on the design of the hands can be relaxed, and thus various designs can be proposed.